CCR1 and CCR2 co-expression on monocytes is nonredundant and delineates a distinct monocyte subpopulation. [RNA-Seq]

The interactions between chemokines and their receptors, particularly in the context of inflammation, are complex with individual receptors binding multiple ligands and individual ligands interacting with multiple receptors. In addition, there are numerous reports of simultaneous co-expression of multiple inflammatory chemokine receptors on individual inflammatory leukocyte subtypes. Overall, this has previously been interpreted as redundancy and proposed as a protective mechanism to ensure that the inflammatory response is robust. In contrast we have hypothesised that the system is not redundant but exquisitely subtle. Our interests relate to the receptors CCR1, CCR2, CCR3 and CCR5 which, together, regulate non-neutrophilic myeloid cell recruitment to inflammatory sites. Here we demonstrate that, whilst most murine monocytes exclusively express CCR2, there is a small subpopulation, which is expanded during inflammation, which co-expresses CCR1 and CCR2. Combinations of transcript and functional analysis demonstrate that this is not redundant expression and that co-expression of CCR1 and CCR2 marks a phenotypically distinct population of monocytes characterised by expression of genes otherwise typically associated with neutrophils. Single cell RNA sequencing confirms this as a monodisperse population of atypical monocytes. This monocytic population has been previously described as having immunosuppressive activity. Overall, our data confirm combinatorial chemokine receptor expression by a subpopulation of monocytes but demonstrate that this is not redundant expression and marks a discrete monocytic population. Bulk RNA-seq of Monocytes isolated from mouse bone in a resting or inflamed state.

趋化因子与其受体之间的相互作用（尤其是在炎症情境下）极为复杂：单个受体可结合多种配体，而单个配体也可与多种受体结合。此外，已有大量研究报道，单一炎症白细胞亚型的表面可同时共表达多种炎症趋化因子受体。既往学界曾将此类现象归因于功能冗余，并将其视作确保炎症反应强度充足的保护性机制。与之相反，本研究提出假说：该系统并非存在功能冗余，而是具备极其精细的调控特性。本研究聚焦于CCR1、CCR2、CCR3及CCR5这四类受体，它们共同负责调控非中性粒细胞髓系细胞向炎症部位的募集。本研究证实：尽管多数小鼠单核细胞仅表达CCR2，但存在一小部分亚群——该亚群在炎症过程中会发生扩增——可共表达CCR1与CCR2。转录组与功能联合分析显示，此类表达并非功能冗余的体现；CCR1与CCR2的共表达，标志着一类表型独特的单核细胞亚群，其特征为表达通常仅见于中性粒细胞的基因。单细胞RNA测序（single cell RNA sequencing）进一步证实，该亚群为一群均质化的非典型单核细胞。既往研究曾将此类单核细胞亚群描述为具备免疫抑制活性。综上，本研究数据证实，部分单核细胞亚群可共表达多种趋化因子受体，且此类表达并非功能冗余，而是标志着一类独立存在的单核细胞亚群。本研究对处于静息或炎症状态下从小鼠骨骼中分离的单核细胞开展了批量RNA测序（bulk RNA-seq）。